All speakers have different acoustical and electrical properties.Find the speakers you like before buying or building a box.Alot of manufacturers(sp) make speakers that are designed to work in less than a cubic foot of air space.Kicker brand comes to mind.Any competent car audio retailler should be able to design a enclosure for your car.
The box size has to be matched to the speaker specs. If you don't do that, there's a 1 in a million chance it'll sound right. I'm building a pair of ~1CuFt 13x13x8" sealed boxes for a pair of 12" subs, bolted to a steel rear seat divider with a hidden 200W RMS amp. It'll be as stealthy as I can make it ::
You don't really need plans, just some 3/4" MDF, glue, screws, insulation, and a router. Then all you need are the speakers specs or reccomended box size.
PartsExpress is my favorite source by far, the tech line is staffed by stereo geeks, and they'll give you box plans for a specific driver.
i say make your own. no one is going to sell a box to fit 3 10's in an old mustang unless you get it custom built. should'nt be too hard to do it on your own. order a crutchfield magazine (crutchfield.com) and look up all of the stuff you want. when you get it matched up, go to a best buy or car toys and they will get you everything you need.